[QE-users] Questions on Applying Pressure in VC-MD Simulations
Nam Tran
vnt981 at uowmail.edu.au
Tue Jul 24 10:00:40 CEST 2018
Dear QE Experts,
I am trying to create an amouphous structure (Na2SiO3) using a melting-cooling process. I use "vc-md" for my calculation but the obtained density seems to be incorrect.
In more detail, I start with orthogonal cell containing 138 atoms to maintain the density of 2.6 g/cm^3 [1]. In the melting process, I run the vc-md (NPT) at 5000 K and pressure of 1 atm (0.00 1kbar). After several MD steps the cell shrinks at lot causing the density increases to 4.77 g/cm^3.
Therefore, I wonder I have done something wrong with the input. Maybe, I have misunderstood that "press = 0.001" is something else but not the external pressure acting on the cell.
I appreciate any help and advice
Regards
........................................................................................
INPUT
&CONTROL
calculation='vc-md',
restart_mode = 'from_scratch',
tstress = .true.,
nstep =20000,
dt = 20.0,
outdir='./tmp/',
prefix='NSO',
pseudo_dir='=/pseudo/'
/
&SYSTEM
ibrav = 0,
nat = 138,
ntyp = 3,
ecutwfc=40.0,
ecutrho=160.0,
occupations = 'smearing',
smearing = 'gauss',
degauss = 0.01,
/
&ELECTRONS
conv_thr = 1.d-6,
electron_maxstep = 500,
mixing_beta = 0.7D0,
mixing_mode='local-TF',
/
&IONS
ion_dynamics='beeman',
ion_temperature = 'andersen',
tempw = 5000,
/
&CELL
cell_dynamics='pr',
press = 0.001,
cell_dofree = 'z',
/
ATOMIC_SPECIES
O 15.999 O.pbe-hgh.UPF
Na 22.990 Na.pbe-sp-hgh.UPF
Si 28.085 Si.pbe-hgh.UPF
CELL_PARAMETERS angstrom
17.4419994354 0.0000000000 0.0000000000
0.0000000000 10.0699996948 0.0000000000
0.0000000000 0.0000000000 10.1600000000
K_POINTS gamma
ATOMIC_POSITIONS angstrom
O 14.268993 1.361002 3.297000
O 8.769019 1.382002 3.281000
O 3.069007 1.401002 3.266000
O 14.253031 1.376002 6.496000
....................................
OUTPUT
......................................................................................................
Computing stress (Cartesian axis) and pressure
total stress (Ry/bohr**3) (kbar) P=-5249.70
-0.05197737 -0.00014172 0.00026844 -7646.13 -20.85 39.49
-0.00014172 -0.04974923 0.00007203 -20.85 -7318.36 10.60
0.00026844 0.00007203 -0.00533369 39.49 10.60 -784.61
Entering Dynamics; it = 22 time = 0.02033 pico-seconds
new lattice vectors (alat unit) :
1.000000000 0.000000000 0.000000000
0.000000000 0.577342049 0.000000000
0.000000000 0.000000000 0.319288923
new unit-cell volume = 6600.8893 (a.u.)^3
new positions in cryst coord
................................................................
................................................
Ekin = 122.83179595 Ry T = ****** K Etot = -6172.23708296
new unit-cell volume = 6600.88934 a.u.^3 ( 978.15087 Ang^3 )
density = 4.76597 g/cm^3
[1]. http://www.ilo.org/dyn/icsc/showcard.display?p_version=2&p_card_id=0359
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